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  • Optical Interferometry Measures the Deflection of Carbon-Based Nanosensors

Jul 2008
A commercially available heterodyne interferometer optically measures directly the displacement of carbon-based nanofibres and nanotubes resulting from airborne ultrasound.

Dr. Triantafillos Koukoulas and Dr. Pete D. Theobald, National Physical Laboratory, UK

The reduction in size of modern electromechanical systems — from micro (MEMS) to nano (NEMS) — brings with it decreasing dimensions to be tested and characterized. Thus, the sensitivity requirements of the testing device and methodology are increased.

Carbon nanofibres and nanotubes offer significant opportunities for further research, which may lead to the commercialization of products including sensors and probes to be used in molecular storage, environmental monitoring and nanoelectronics.

Carbon nanofibres in a forest topology are shown in a scanning electron microscope image at 10,0003 magnification.

Ultrasonic acoustics — a field that uses nanotubes and nanofibres — needs further exploitation. Of the various techniques for manufacturing nanoelements, one dubbed plasma-enhanced chemical vapour deposition was explored and fine-tuned by W.I. Milne’s group at Cambridge University in the UK. This method produces vertically aligned arrays and forests of elements.

Acoustic sensors that have significant potential for use in the airborne ultrasonic frequency range (>20 kHz) currently rely on exploiting the piezoelectric property of nanoelements; a vertical deflection produces an electrical output that is characteristic of their motion. Given the dimensions of a typical tube or fibre (5 to 20 μm in length with a diameter of a few hundred nanometres), the low signal-to-noise ratio becomes significant even though displacements in the nanometre region (or less) can be measured.

Optical interferometry (homodyne and heterodyne) is a well-established technique in various scientific fields that provides an alternative method for exploiting the response of carbon nanofibres or -tubes to airborne ultrasonic fields. In particular, a heterodyne interferometer (commercially referred to as a laser Doppler vibrometer in this case) can measure the nanometre deflection of active carbon-based elements.

This graph plots the measured displacement of carbon nanofibres resulting from the presence of the 1-MHz airborne acoustic field from the ultrasonic transducer.

The underlying principle of a heterodyne interferometer/vibrometer is to divide a laser beam (HeNe in this case) into a target beam and a reference beam. A Bragg cell, typically at 40 MHz, frequency-modulates the reference beam isolated within the vibrometer, while the other beam exits the vibrometer to probe a target. If the target is moving, the target beam will be subject to a Doppler shift proportional to the velocity of the reflecting surface. The vibrometer then collects this reflected, Doppler-shifted light and mixes it with the frequency-modulated reference beam.

Differential amplification and demodulation of the mixed signal produces an electrical output that is directly proportional to the velocity of the moving target; therefore, the displacement may be measured. This direct measurement capability that interferometric techniques offer also makes this a potential method for the calibration of sensors exploiting the piezoelectric properties of the nanoelements.

This graph shows the measured displacement produced at the surface of the transducer generating a 1-MHz airborne acoustic field.

By applying an airborne ultrasonic field incident on the nanoelements — using an ultrasonic transducer in close proximity (typically a few millimetres) — it is possible to measure the deflection of the nanoelements that results from the acoustic wave by aligning the target beam of the vibrometer to reflect off the structures at an optimum angle. The higher attenuation coefficient in air at ultrasonic frequencies compared with audible frequencies necessitates such close proximity to the transducer and results in the measurements in the acoustic near-field.

A comparison of the deflection displacement of the nanofibres and the vibration displacement measured at the surface of the transducer shows that the nanofibre displacements are around seven times lower than the displacement generated by the transducer. This can be attributed to the high attenuation coefficient and to the response characteristics of the nanofibres to the acoustic field. In both cases, the measured displacements lie in the nanometre region.

A high signal-to-noise ratio is observed for the vibrometer measurements, demonstrating that optical methods can be employed to measure the response of carbon nanofibres to airborne ultrasonic fields. In fact, interferometry may be utilized for the deflection measurement of nanoelements for any applica-tion that causes deflections in the nanometre region.

Looking to the future, the main objective is to produce nanofibres that exhibit a more sensitive response to acoustic fields with frequencies below 20 kHz for measurement in the audible range. This will create exciting opportunities for high-fidelity, absolute measurement of audible acoustic fields with high spatial resolution using optical methods. Using interferometry also may provide a mechanism for the calibration of future cochlear implants based on piezoelectric carbon nanotube technology.

Contact: Triantafillos Koukoulas, National Physical Laboratory; fax: +44 20 8943 6217;
e-mail: Pete D. Theobald, National Physical Laboratory;

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